Throughout the world, animals are routinely being shipped from farm to farm. Animals are also regularly imported or exported across state and national borders. Although these animals may appear clinically healthy with no signs of disease, the possibility exists that many of these animals may harbor potentially serious viral infections. Such animals may later become ill with a disease. More seriously from an economic point of view is that many asymptomatic carriers of viral infections may never show signs of disease but yet will continue to shed virus to other animals of a herd or flock. These asymptomatic carriers of viral infections, upon being introduced into a new herd or flock, present a significant risk of infection to the other animals in their new environment.
At present, there are a small number of tests that are routinely performed on apparently healthy animals to ensure that they will not present a threat in their new surroundings. Most notable of these tests is the Coggins test which is performed on horses prior to shipping to ensure that the horse is not infected with Equine Infectious Anemia, a disease which is transmitted from one horse to another by blood sucking insects. Such tests, however, are specific for particular viral diseases and, even if it were possible, it would be economically and physically impractical to test animals for every known viral disease prior to shipping. Moreover, tests are unavailable for the detection of most viral diseases.
Accordingly, the viral infection status of animals is not known before shipping. Yet, knowledge of the viral-infection status of asymptomatic shipped animals is probably the most critical aspect in preventing disease outbreaks. There is a critical need for a means for determining viral infection status of animals before they are shipped.
Knowledge of viral infection status is also critical in the case of an existing outbreak of disease. In recent years several outbreaks of highly contagious viral diseases have occurred.
Most notable among these is the foot-and-mouth disease (FMD) outbreak that began in 2001 in England. This outbreak affected thousands of farms and thousands of animals were found to be infected with the FMD virus.
In outbreaks like these, which are typically due to viruses, animals that are suspected of having been infected with the virus are slaughtered in order to control the disease. The slaughtered animals are then tested to determine if they did, indeed, harbor the virus. Because asymptomatic animals from the farm of the virus-positive animal may have been exposed to the virus, these other animals are likewise destroyed.
This slaughter of asymptomatic, possibly exposed animals is done as a precaution because there presently are insufficient tests to determine whether the animals have been infected. In a great many of the cases, it is possible that most if not all of the slaughtered animals were not infected, and so truly posed no danger of spreading the disease. Unfortunately, because of the lack of definitive testing to determine whether or not the animals are virally infected, the slaughter program is a necessary step towards preventing the further spread of the virus.
In addition to the lamentable and unnecessary loss of animal life, such non-discriminating slaughter programs are extremely disruptive to the farmer who loses all of his livestock and to the agricultural economy of the region or the country. Moreover, the costs of such slaughter programs are high and include compensation of the farms for the destruction of the animals. Such compensation, however, is typically insufficient to truly recompense the farmer as it often takes several years to rebuild a lost herd and to make the farm economically viable once more.
A significant need exists for a method to screen asymptomatic animals suspected of being infected with a virus to determine whether or not the animals are infected.
A similar need exists relating to the movement of people throughout the world. In the world of today, people move from one country to the next with little or no knowledge as to whether or not people are harboring potentially lethal viral infections. Although it may never be practical to routinely test all people for infection before travel, if a test for viral infection existed, people that are traveling from a country that is experiencing an outbreak of a viral disease could be tested. Such testing could be used, for example in the case of ebola virus, to prevent the spread of the terrible disease caused by this virus to countries where the virus does not presently exist. Accordingly, as with animals, a significant need exists for a test that can be used to determine viral-infection status in humans.
Mx proteins are monomeric GTPases, which, depending on the species of animal and type of virus, are potent inhibitors of viral replication (Samuel, Virology 183:1-11 (1991)). The sequences of Mx proteins from various species, including sheep, cattle, pigs, and horses, are publicly available through GenBank and have been assigned GenBank Accession numbers X66093, U88329, M65087, and U55216, respectively. Although the antiviral effects of Mx are generally directed against negative-stranded RNA viruses (e.g. orthormyxovirus), their expression is induced in all cells that possess Type I interferon (IFN) receptors.
It has been reported that the gene for Mx protein does not primarily respond to viral infections, but rather is secondarily induced in response to an elevation in virus-induced IFN. Bazzigher, L., et al., Virology, 186:154-160 (1992). Elevations in Mx protein are present in both acute and chronic viral infections. Fernandez, M., et al., J. Infectious Diseases, 180:262-267 (1999). Induction of Mx protein has been used with patients suffering from an infection to determine whether the illness was due to a viral or bacterial infection. Halminen, M, et al., Pediatric Research, 41(5):647-650 (1997); Forster, J., et al., Acta Paediatr., 85:163-167 (1996); Chieux V., J. Virological Methods, 70:183-191 (1998), and Haller et al., Rev. Sci. Tech. 17:220-230 (1998), and U.S. Pat. Nos. 5,198,350 (Horisberger) and 6,180,102 (Hanai). Determinations of Mx protein have been utilized as a method for determining levels of interferon in patients known to be suffering from an infectious disease. U.S. Pat. No. 6,200,559 (von Wussow); von Wussow, P., et al., AIDS, 4(2):119-124 (1990), Nieforth, K A, et al., Clinical Pharmacology & Therapeutics, 59(6):636-646 (1996); and Oh, SK, J. Immunological Methods, 176:79-91 (1994). Mx protein has also been used as a marker for interferon production to determine the response to vaccination. Roers, A., et al., J. Infectious Diseases, 169:807-813 (1994). It has also been reported that Mx protein levels are elevated in illnesses due to autoimmunity. Rump, J A, Clin. Exp. Immunol., 101:89-93 (1995).
The prior art thus discloses elevation in levels of Mx protein in patients showing signs of a disease or in subjects that were known to be exposed to a virus or to a vaccine. There is no indication in the prior art that Mx protein determination may be useful as a diagnostic tool in an animal free of signs of a viral disease, which animal has not recently been knowingly exposed to a virus or been recently vaccinated.